The exchange of cholesterol between human erythrocyte membranes and three species of plasma lipoproteins, low density lipoproteins and the two major fractions of high density lipoprotein, HDL2 and HDL3, was studied. With all three lipoproteins, and over the temperature range 5 degrees C - 37 degrees C, exchange rates first increased with lipoprotein concentration, and then reached a limiting value at higher lipoprotein concentrations. This limiting exchange rate was the same for all lipoprotein species, and their mixtures, and has an activation energy of about 9 kcal. It is believed that the limiting rate is due to a slow process occurring within the cell membrane, possibly the diffusion of cholesterol. The cholesterol exchange between erythrocytes and lipoproteins in normal human serum would be predicted to take place at the limiting exchange rate.